1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device with reduced unevenness in brightness of a liquid crystal display screen with point-sequential driving.
2. Description of the Background Art
Referring to FIG. 11, a signal processing circuit 122 receives an input of a digital image signal and performs signal processing on the signal for output to a signal line driving circuit 101 as an image signal. A timing circuit 121 that receives and sends signals to and from the signal processing circuit 122 receives an input of a synchronization signal of a signal line 105 and a scanning line 106 as well as a digital clock signal, and processes them for output to a scanning line driving circuit 102 as a scanning line signal and simultaneously outputs a synchronization signal to the signal line driving circuit 101. The signal line driving circuit 101 applies an image signal voltage via signal lines 105 to a liquid crystal display element (not illustrated) at intersections with a scanning line 106 in an on-state by point-sequential driving. The on-state of the scanning line 106 moves sequentially from the top of the display screen in a downward direction. Typically, a display region 110 is divided into a plurality of blocks 111, and the signal lines 105 and the signal line driving circuit are also divided into blocks.
Referring to FIG. 12, the display region 110 as well as the signal lines are divided into four blocks 111. The scanning line driving circuit 102 applies a scanning signal voltage selectively to each scanning line to open and close a pixel switching element. The signal line driving circuit 101 applies an image signal voltage to a liquid crystal display element via a pixel switching element in an on-state for driving the liquid crystal display element. When point-sequential driving is carried out in a liquid crystal display device, a transient phenomenon with a time constant of RC occurs due to the resistance (R) and the capacitance (C) in each liquid crystal display element, whereby the signal voltage is not applied without change to the liquid crystal display element of each pixel. When the image signal voltage is applied to the liquid crystal display element by the signal line driving circuit 101, the voltage applied to the liquid crystal display element rises in a transient manner with the time constant RC due to the transient phenomenon. As a result, in the case of applying a voltage to one scanning line in each block 111 in point-sequential driving, the actual voltage of the liquid crystal display element of the pixel to which the voltage has been applied first is higher than the voltage of the liquid crystal display element of the pixel to which the voltage has been applied last. Regarding the voltages of the liquid crystal display elements, the voltage at the end of the point-sequential driving of one scanning line in a block is frozen as it is, so that unevenness of brightness such as shown in FIG. 12 occurs. Referring to FIG. 12, the direction 115 of point-sequential driving in each block 111 of the display region 110 is the same. Therefore, a strong unevenness of brightness occurs at a boundary of the blocks 111 on the display screen.
In order to prevent a strong unevenness of brightness occurring at the boundary of blocks, a proposal has been made in which the driving direction 115 of point-sequential driving of each block is made opposite to the driving direction 115 of the adjacent blocks, as shown in FIG. 13 (Y. Aoki et al., A 10.4-in. XGA Low-Temperature Poly-Si TFT-LCD for Mobile PC Applications, SID 99 DIGEST, pp. 176-179). By this point-sequential driving method, the strong unevenness at the boundary of blocks is eliminated.
However, although the unevenness of brightness at the boundary of blocks is eliminated by the aforementioned point-sequential driving method, the unevenness of brightness within each block still remains without being eliminated. If a portion having a brightness lower by 3% or a portion having a brightness higher by 3% than the brightness of the screen is present, the presence of unevenness in brightness is recognized by a human eye. Therefore, the unevenness of brightness caused by the same voltage difference is recognized more keenly on a dark screen than on a bright screen. If such an unevenness of brightness is conspicuous, the display quality is deteriorated to a considerable extent, so that an improvement must be made in order to prevent the unevenness of brightness from being conspicuous.
An object of the present invention is to provide a liquid crystal display device that performs point-sequential driving so that the unevenness of brightness on the display screen will not be recognized to be conspicuous.
A liquid crystal display device according to the first aspect of the present invention includes a plurality of scanning lines and a plurality of signal lines that make an intersection with each other in a display region divided into a plurality of blocks by a boundary parallel to the signal lines; liquid crystal display elements disposed at sites of the intersection; a signal line driving circuit that applies an image signal voltage sent from a signal processing circuit and a timing circuit to the signal lines divided into a plurality of blocks for performing point-sequential driving of the signal lines; and a scanning line driving circuit that applies a scanning signal voltage sent from the timing circuit to the plurality of scanning lines for driving the scanning lines, the signal line driving circuit and the scanning line driving circuit being disposed in a driving circuit region, wherein the above-mentioned signal line driving circuit includes a driving direction switching circuit for inverting a driving direction of the point-sequential driving in the blocks according to a signal from the signal processing circuit, and the above-mentioned signal processing circuit includes an image signal rearranging circuit for performing rearrangement of image signals, which is needed in accordance with inversion of the driving direction, in synchronization with the inversion of the driving direction.
By this construction, the pattern of unevenness in brightness in the display region can be made uniform by changing the arrangement of unevenness in brightness with lapse of time in each block. As a result, it is possible to reduce the extent of recognized unevenness in brightness not only at the boundary of the blocks but also within the blocks.
In the liquid crystal display device according to the first aspect of the present invention, the timing circuit includes a driving direction switching timing output circuit for outputting a switching timing of the driving direction to the signal line driving circuit.
By this construction, the driving direction switching circuit can perform inversion of the driving direction of the point-sequential driving, for example, for each frame or for each line. Here, one frame refers to the period of time for applying a scanning signal voltage to each scanning line from the top to the bottom of the screen while performing the point-sequential driving in each block until the point-sequential driving is finished, or the screen displayed at that time. In other words, one collective screen is displayed over the entire display region within the period of time for one frame. As a result, thickly displayed parts are replaced with thinly displayed parts for each frame, so that the unevenness is averaged in time, so that the unevenness of brightness is less noticeable.
Further, with the use of the timing circuit and the signal processing circuit, the device can be operated such that, for example, the direction of point-sequential driving is alternated line by line in a block, and the driving direction of each line is inverted for each frame. According to the above-mentioned construction, the unevenness in brightness is disposed in alternate arrangement for each scanning line in the block, so that the unevenness is intermixed in fine spatial units. As a result, the unevenness in brightness is not recognized to be conspicuous, and the displayed image is recognized to be uniform by a human eye. Furthermore, in the case of alternately inverting the polarity of the applied voltage every time the driving direction is inverted for each scanning line, it is possible to avoid a situation in which the whole block is in a state of positive voltage or negative voltage. As a result, the flickering or the like can be prevented.
A liquid crystal display device according to the second aspect of the present invention includes a plurality of scanning lines and a plurality of signal lines that make an intersection with each other in a display region divided into a plurality of blocks by a boundary parallel to the signal lines; liquid crystal display elements disposed at sites of the intersection; a signal line driving circuit that applies an image signal voltage sent from a signal processing circuit and a timing circuit to the signal lines divided into a plurality of blocks for performing point-sequential driving of the signal lines; and a scanning line driving circuit that applies a scanning signal voltage sent from the timing circuit to the plurality of scanning lines for driving the scanning lines, the signal line driving circuit and the scanning line driving circuit being disposed in a driving circuit region, wherein the above-mentioned blocks include a plurality of first subblock groups and a plurality of second subblock groups which are alternately disposed, and the signal line driving circuit includes a first group driving circuit for applying the image signal voltage to the liquid crystal display elements of the first subblock groups for performing point-sequential driving, and a second group driving circuit for applying the image signal voltage to the liquid crystal display elements of the second subblock groups for performing point-sequential driving. At this time, as in the liquid crystal display device of another second aspect of the present invention, the signal processing circuit and the timing circuit are preferably constructed in such a manner that the driving directions in the first subblock groups and the second subblock groups are opposite to each other.
By this construction, each block is divided into smaller parts by a boundary in the signal line direction. For example, adjacent to a thickly displayed part of the first subblock group on the left end in a block, a thinly displayed part of the second subblock group is disposed. An opposite combination of unevenness in brightness is disposed on the right end in the block. Further, at the center of each block, parts displayed in an intermediate thickness of the first subblock group and the second subblock group are alternately disposed. As a result, it is possible to obtain a displayed image in which the unevenness in brightness is not conspicuous. However, in this case, unevenness in a longitudinal stripe pattern remains because the boundary in the signal line direction is present. The above has been explained for the first subblock groups and the second subblock groups. However, the same applies even if the third subblock groups, the fourth subblock groups, etc. are added.
In the liquid crystal display device according to the second aspect of the present invention, the first group driving circuit and the second group driving circuit include a first group driving direction switching circuit and a second group driving direction switching circuit, respectively, for inverting the driving direction in the first subblock groups and the second subblock groups, and the signal processing circuit includes an image signal rearranging circuit for performing rearrangement of image signals in each subblock group, which is needed in accordance with switching of the driving direction, in synchronization with the inversion of the driving direction.
By this construction, uniformization of the unevenness in brightness in time can be made in addition to the above-mentioned spatial uniformization of the unevenness in brightness by intermixing of the unevenness in brightness in minute units in the above-mentioned liquid crystal display device according to the second aspect of the present invention. As a result, the unevenness in brightness will be less noticeable, and the unevenness in brightness in a longitudinal stripe pattern will also be less noticeable.
In the liquid crystal display device according to the second aspect of the present invention, the timing circuit preferably includes a driving direction switching timing output circuit for outputting a switching timing of the driving direction to the signal line driving circuit.
By this construction, the device can be operated so that the driving direction is inverted for each frame or for each scanning line. As a result, the unevenness in brightness is made uniform in time while the unevenness in brightness is intermixed in a checker-board pattern spatially divided into very small parts. Therefore, the unevenness in brightness is hardly recognizable by a human eye, and it is possible to obtain a displayed image being extremely excellent in uniformity. Further, the flickers are hardly visible if the image signal voltage is inverted when the driving direction is inverted for each scanning line.
In the liquid crystal display device according to the first and second aspects of the present invention, in all the liquid crystal display devices having a driving direction switching circuit that inverts the driving direction, the driving direction switching circuit includes a polarity inversion circuit for inverting the polarity of the image signal voltage every time the inversion of the driving direction of the point-sequential driving is carried out.
By this construction, it is possible to prevent an image signal voltage of the same polarity from being applied over the entire block. As a result, flickers and others on the screen can be restrained.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.